Light amplifier stages

ABSTRACT

The present invention concerns light amplifier stages in which the provision of a fully reflecting surface at the end of a conventional amplifier stage causes it to produce an amplifying effect equivalent to plural conventional stages.

United States Patent Michon 51 June 6, 1972 [54] LIGHT AMPLIFIER STAGES[561 Reterences Cited [72] Inventor: Maurice Michon, Draveil, FranceUNITED STATES PATENTS Assignee: Compagnie Generale D'Electflcite Pariset al -.330/4.3 France 3,514,709 5/1970 Jacob ..330/4.3 3,407,36410/1968 Turner ..330/4.3 1 Flled= 24, 1970 3,533,015 10/1970 Wuerker....331/94.s [21] AppL No; 31,634 3,518,436 6/1970 De Mana et al..33l/94.5

' Primary Examiner-Malcolm F. Hubler [30] Foreign Application PriorityData Assistant Examiner-N. Moskowitz A: S h ,R th ll,M' M k Apr. 24,1969 France ..69l3l 14 tome) me o we Zmn & acpea [57] ABSTRACT [52] US.Cl ..330/4.3, 331/945 [51] Int. Cl. ..H0ls 3/05 The mvemlo concerns "8amphfler Stages whch 58 Field of Search ..330/4.3- 332/7.51- 33 1194.5Pwvisin a fully reflecting Surface the end a ventional amplifier stagecauses it to produce an amplifying effect equivalent to pluralconventional stages.

2 Clalrns, 3 Drawing Figures W LIGHT AMPLIFIER STAGES BACKGROUND OF THEINVENTION The present invention concerns laser light pulse amplificationsystems and more particularly the amplifier stages of these systems.

A laser amplifier system generally comprises a pilot laser, emitting alight pulse, associated with amplifier stages which enable, startingfrom the pulse emitted by the pilot laser, a high-power pulse to beobtained.

The pilot laser generally consists of an active laser bar associatedwith its pumping means, for example a discharge tube wound in a helicalcoil. The bar is arranged between two mirrors of a Perot-Fabry resonantcavity, one of the mirrors being semitransparent to enable one part ofthe laser emission to emerge from the cavity and the other being fullyreflecting and capable of being driven in a rotational movement so as toobtain a triggered laser. The photon flux being emitted by the pilotlaser penetrates into a bar deprived of its resonant cavity, forming thefirst stage of the amplifier system. The active medium, formed by a barpumped, for example, by the light flash of a discharge lamp, produces anamplification of the pilot laser emission. This arrangement thus causesan appreciable power gain. Several amplifier stages can thus be arrangedin series, the stimulated emission in the following stages beingprovoked by the photon flux coming from the preceding stages.

For medium-power amplifier systems, bars all having the same diameterand approximately the same length are used.

In lasers such as those described above, the light pulse passes onlyonce through the active medium of each amplifier stage and all thestimulated mediums such as ions or molecules do not assist in producingthe stimulated emission, thus a loss of the total output of the systemensues. Moreover, a laser system having a pilot laser and, for example,four amplifier stages, occupies a length of about three to four meters.

In connecting the pilot laser and the stages in series, it is essential,to avoid their being submitted to vibrations, to arrange them on anoptical bench whose length is at least the same as that of the system.Such an optical bench generally consists of a single granite bar whosecross section is substantially a square about 1 centimeter which is verycostly.

SUMMARY OF THE INVENTION The object of the present invention is toovercome all of these disadvantages.

The present invention concerns a stage of a light pulse amplifier systemcharacterized more particularly in that it comprises, arranged in thesame light pulse propagation path and successively, starting from theinput of the said stage, a selective two-state polarizer which isconductive or deflects a light pulse, depending upon the direction ofits polarization plane, an active laser bar associated with its pumpingmeans, a fully reflecting surface, and between the said selectivepolarizer and the said totally reflecting surface, a quarter-wave plate.

Other characteristics and advantages of the present invention willbecome apparent during the following description in relation to theannexed drawing as an illustrative example which is in no way limiting,in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an amplifier systemaccording to prior art; and

FIGS. 2 and 3 show two embodiments of a laser amplifier systemcomprising an amplifier stage according to the invention.

In FIG. 1, an amplifier system illustrating prior art is shown.

' DETAILED DESCRIPTION OF THE INVENTION The amplifier system comprises,on a same optical axis, successively, a triggered pilot laser 1, a pulsesplitting system 2 for reducing the duration of the pulse emitted by thepilot laser, which is generally in the order of 20 ns, to approximately1 to 2 ns, and several amplifier stages such as those seen at 3 and 4.

The pilot laser 1 can consist, for example, of an active laser bar 5surrounded by its discharge tube 6 arranged between a semitransparentmirror 7 and a fully reflecting prism 8 which can be driven in arotational movement.

At the output of the pilot laser 1, the amplifier system comprises thepulse splitting system 2, which in itself, is known, comprisinggenerally an electro-optical dephasing cell 9 such as, for example, aPockels cell arranged between two crossed polarizers 10 and 11 which aregenerally Gian prisms.

The electro-optical cell can be controlled by a very short voltage pulsegenerated by a voltage generator 12.

The amplifier stages, which can be identical, consist of an active laserbar 13 associated with pumping means 14 which can be, for example, adischarge tube.

The operation of the system described above is as follows:

The pilot laser generates a light pulse which, when triggered by aprism, cannot go below 20 us. The duration of this pulse is brought to avalue in the order of a nanosecond by the pulse splitting system 2,whose operation is well known.

The group of photons which corresponds to the short-duration pulse,produces, on entering the first laser bar 13 of the stage 3 of theamplifier system, a first stimulated emission, then the amplified pulseleaving this first stage penetrates the bar of the second stage toproduce further stimulated emission, and it is the same for all thestages of the amplifier system.

Thus, a pulse which has been appreciably amplified is obtained at theoutput of the system.

On the two FIGS. 2 and 3, showing two embodiments of an amplifier stageaccording to the invention, the same items bear the same referencenumbers.

FIG. 2 shows a first embodiment of an amplifier system comprising anamplifier stage according to the invention associated with a pilotlaser, this system being equivalent to a system having two distinctamplifier stages. The system comprises a pulse generator such as, forexample, a pilot laser 15 triggered by a prism, associated with a pulsesplitting device 16 comprising an electro-optical dephasing cell 17,such as for example, a Pockels cell, arranged between two crossedpolarizers 18 and 19, the said pulse generator being arranged on thesame optical axis, and in the direction of progression of a light-pulse.The crossed polarizers are devices generally consisting of opticalcrystals through which pass light beams which are rectilinearlypolarized in a certain direction in relation to the optical axes of thecrystals, but deflect rectilinearly polarized light beams whosepolarization plane has a direction at right angles to the aforementionedlight beams. These polarizers or other similar systems will be definedin the description as: selective polarizers having a conductive stateand a deflecting state, depending upon the direction of the polarizationplane of the light cooperating with them. The said two states whichcorresponds to two orthogonal directions of the light polarizationplane. As previously, the electrooptical cell is controlled by a voltagepulse supplied by a generator 20.

An active laser bar 21 associated with its pumping means 22, forexample, a flash lamp wound helically round this bar 21, and a fullyreflecting surface 23 is arranged at the output of the pulse splittingdevice. For example, a quarter-wave plate 25, enabling the polarizationplane of a wave to be rotated by (qr/2) when this wave passes throughthe plate 25 twice, is arranged between the output face 24 of the bar 21and the fully reflecting surface 23.

In this example, the quarter-wave plate is arranged between the bar andthe fully reflecting prism, but it can also be arranged between thepolarizer and the bar.

The operation of the system is as follows:

The duration of the light pulse generated by the pilot laser 17 isshortened by the pulse-splitting system 16 and the short pulse leavingthe pulse-splitting system through the polarizer 19 is polarizedrectilinearly, then it enters the bar 21 which is pumped by the flash ofthe flash lamp 22. This short pulse is thus amplified a first time.Then, after reflection on the prism 23, the pulse again crosses the barwhich is still partially stimu lated but having, in the process, crossedthe quarter-wave plate 25 twice, this having resulted in the rotation by(1r/2) of the polarization plane. When the light leaves the bar throughthe face 26 of the bar 21 and falls onto the polarizer 19, the lightwhose polarization plane direction crossed by the con ductive directionof the polarizer is deflected from the axis of the system and emergesfrom the latter at 27.

Therefore, according to the device described above, an amplifier systemalmost equivalent to a system comprising two amplifier stages, since thepulse has twice crossed a stimulated active bar, has been obtained.

As the light pulse crosses the same bar twice, a maximum of pumpingenergy has been converted into amplifying energy and has contributed toincreasing the output of a single amplifier stage.

In this embodiment, the amplifier stage which is twice crossed by alight pulse to be amplified consists of the polarizer 19, the activemedium 21, the quarter-wave plate 25 and the prism 23, but in the casewhere the amplifier system comprises a pulse-splitting system consistingof an electro-optical cell arranged between two crossed polarizers, theoutput polarizer of the pulse-splitting system can coincide with thepolarizer which is built into the amplifier stage, this being one of theadvantages of the invention.

FIG. 3 shows a second embodiment of an amplifier system comprising anamplifier stage according to the invention which is substantiallyequivalent to a system having four amplifier stages.

The system according to FIG. 3 comprises the same elements as before,but a fully reflecting mirror 28 is arranged in the path of the lightbeam deviated at 27 by the polarizer 19.

The operation of that amplifier system is as follows:

The first phase of operation is, in all respects, identical to theoperation as described above in the first embodiment. The light pulse isdeviated by the polarizer 19 and sent through the bar 21 after havingbeen reflected on the mirror 28 and having been deviated once again bythe polarizer 19, the light pulse plane of polarization having nodefinite direction.

The pulse therefore crosses the bar twice more; it also crosses thequarter-wave plate 25 twice more, its plane of polarization thereforeundergoes a further rotation by (17/2) and consequently, the pulseemerging for the second time from the bar 21 through the face 26 crossesthe polarizer 19, without being deflected, as well as theelectro-optical cell 17,

since during that time, the voltage pulse controlling that cell hasfallen back to its zero value. The pulse arriving on the selectivepolarizer 18 is deflected from its path to be ejected from the system at29 since this polarizer 18 is crossed in relation to the polarizer 19.In this case, an amplifier system substantially equivalent to a systemcomprising four stages in series and an increase in the output of asingle stage have been obtained.

Of course, the invention is in no way limited to the embodimentdescribed and illustrated, which has been given only by way of example.More particularly, details may be modified and certain arrangements maybe changed or certain means may be replaced by equivalent means.

I claim:

1. An amplifier stage for use in a light amplification system, whereinthe light to be amplified consists of a beam of light generated in adirection along a first axis, said amplification stage comprising:

a. a first selective polarizer means positioned along said first axisfor conducting, in a direction along said first axis, light polarized ina first plane and for deflecting, in a direction along a second axis,light polarized in a second plane wherein said first and second planesare perpendicular;

b. an electrooptical dephasing cell positioned along said first axis;

c. a second selective polarizer means positioned along said axis forconducting, in a direction along said first axis, light polarized insaid first plane and for deflecting, in a direction along a third axis,light polarized in said second d. a btive laser bar and a pumping meansassociated therewith positioned along said first axis;

e. a first fully reflective means positioned along said first axis forreflecting a beam from said laser bar back through said laser bar in theopposite direction along said first axis; and

f. a second fully reflective means positioned along said third axis forreflecting light deflected by said second selective polarizer means inthe opposite direction along said third axis.

2. An amplifier stage as recited in claim 1 in which saidelectro-optical cell is a Pockels cell.

I! i I i

1. An amplifier stage for use in a light amplification system, whereinthe light to be amplified consists of a beam of light generated in adirection along a first axis, said amplification stage comprising: a. afirst selective polarizer means positioned along said first axis forconducting, in a direction along said first axis, light polarized in afirst plane and for deflecting, in a direction along a second axis,light polarized in a second plane wherein said first and second planesare perpendicular; b. an electro-optical dephasing cell positioned alongsaid first axis; c. a second selective polarizer means positioned alongsaid axis for conducting, in a direction along said first axis, lightpolarized in said first plane and for deflecting, in a direction along athird axis, light polarized in said second plane; d. an active laser barand a pumping means associated therewith positioned along said firstaxis; e. a first fully reflective means positioned along said first axisfor reflecting a beam from said laser bar back through said laser bar inthe opposite direction along said first axis; and f. a second fullyreflective means positioned along said third axis for reflecting lightdeflected by said second selective polarizer means in the oppositedirection along said third axis.
 2. An amplifier stage as recited inclaim 1 in which said electro-optical cell is a Pockels cell.